Art of coating aluminum



March 15, 1938. F, A, WALES 2,111,377

ART oF GOATING ALUMINUM Filed Nov. 14, 1935 2 Sheets-Sheet. l

A TTORNEY.

Filed Nov. 14, 1935 2 Sheets-Sheet. 2

Inn un "uuu INVENITOR. ai /Z Mles.

ATTORNEY.

Patentedl Mar. 15, 1938 UNITED STATES PATENT OFFICE ART, oF ooA'rlNGALUMINUM Fred A. Wales, Detroit, Mich. Application November-14, 1935,serial No. 49,647

3 Claims.

'Ihe present invention relates to the art of coating aluminum oraluminum alloys or articles formed therefrom, and particularly toimprovements in the method of and apparatus for 5 forming such coatings,particularly coatings of the so-called "oxide type.

Such coatings on aluminum or aluminum alloys are commonly recognized asbeing formed chiefly of A1203, S03, and H2O, and are formed as a 1nplurality of laminated layers which vary. as to thickness, hardness andporosity. Such coatings have been formed by processes heretofore knownto the art by making the aluminum or aluminum alloy an anode in anelectrolytic cell having sulphuric acid asthe electrolyte. The coatingsare formed upon the passage of direct current electrical energiesthrough the cell and are formed on the aluminum or the aluminum alloyswhich form the anode in the cell. The direct current used in suchprocessesI is of the order of twelve to eighteen volts and of from sixto twenty amperes per square foot of material undergoing treatment. Acathode,` electrically connected in the circuit, forms the other contactwith the electrical circuit and is placed in the electrolyte. In certainembodiments, such cathode frequently was the tank, and the current owedbetween the cathode and the anode through the electrolyte and caused thegrowth of the sor-called oxide'coating at the anode. In using suchprocesses for the formation of the oxide coatings on aluminum oraluminum alloy articles, 'such for example as pistons for use ininternal combustion engines, I have discovered that the coating formedby the processes of the prior art was not'satisfactory in such fieldsdue particularly to the large variations in neness of grain structure,the irregularity asto size and distribution of p'ores, the extremevariations in hardness between the re- 0 spective layers, and therelatively high` cost of `processing and of the apparatus for usetherewith.

The process and apparatus of the present invention has as its principalobject to provide a coating for aluminum or aluminum alloys, or articlesformed therefrom, the coating being superior to coatings previouslyknown and characterized by having a very dense ne grained laminatedstructure. The various layers exhibit substantially uniform hardnesscharacteristics and are permeated with very small and substantiallyuniform pores between the particles of the coat- 'It is a'further objectof the present invention to provide a process and apparatus forproducing a protective coating on aluminum or aluminum alloys, orarticles formed therefrom, which is particularly, but not exclusively,adapted to pro-` duce the improved coatings on. pistons formed ofaluminum or aluminum alloys -for use in in- 5 ternal combustion enginesor similar purposes.

A `further object of the present invention is to provide a process forthe production of protective coatings on aluminum or aluminumalloys, orarticles formed therefrom, in which the cost 10 of operation and of theequipment has been reduced over similar costs of prior art processes andequipment, and which, while being more economical than processes andequipment of the prior art, produces coatings which are superior 15 A tothe coatings produced by the prior art.

It is a further object of the present invention to provide an apparatusfor use in forming protective coatings on aluminum or aluminum alloys,or articles formed therefrom, which is a portable self-contained unit.

Other objects of the present invention will appear 'in the followingdescription and appended claims. One preferred form of apparatus em.bodying the present invention is shown by Way of example Vin theaccompanying drawings forming a part ofthis specication wherein likereference characters designate corresponding Darts in the several views,and in which Fig. 1 is a view in perspective of an apparatus 30 which isadapted to be used in connection with the present process and embodiedin the form of a self-contained portable unit.

Fig. 2 is a front view in elevation of the apparatus of Fig. 1 showingthe enclosing cover in 35 closed position.

Fig. 3 is an end view in elevation of the apparatus shown in Fig. 2.

. Fig. 4 is a sectional view taken along the line 4-4 of Fig. 2 in thedirection of the arrows.

Before explaining in detail the present invention it is to be understoodthat the invention is not limited in its application to the details ofconstruction and arrangement of parts illustrated in the accompanyingdrawings, since the invention is capable of other embodiments and ofbeing practiced or carried out in various Ways. Also, it is to beunderstood that the phraseology or terminology employed herein is forthe purpose of descriptionv and not of limitation, and it 50 is notintended to limit the invention claimed herein beyond the requirementsof the prior art.

The apparatus, shown in the drawings com- `prises a casing I0 which ismounted upon a supporting rack Il, which may be provided with aplurality of casters I2 to permit moving the en.-

tire unit from place to place about a plant. The

casing I0 is divided interiorly to form a rinse tank I3 anda tank I4,which is provided with a lead or other non-corrosive lining I5 and isadapted to contain the electrolytic bath.

Mounted on one end of the framework II is a bracket I6 on which ismounted an electric motor I1 which drives afan I 8 of the blower type.An air pump IB, preferably of the pulsating type, is also driven by themotor Il and is connected by means of the conduit 2l) with a series ofperforated pipes 2I yextending across they bottom of the lead-lined tankI4. Air is supplied to the pipes 2| and is discharged therethrough tokeep the electrolyte agitated during operation of the apparatus. The fanor blower I8 is connected by means of a duct 22 with the hooded upperpart of the tank I4 (Fig. 4) and serves to ventilate the apparatus whilein operation.

During the operation of the apparatus, a hood or cover 23 isplaced inthe closed position, as shown in Fig. 4. 'I'he cover 23 prevents fumesgiven olf during the carrying out of the process from escaping into theatmosphere, thus functioning as a collection hood from which the'collected fumes are drawn through the duct 22l and are thereafterdischarged through a stack (not shown), which is connected with adischarge opening 24 on the blower I8. i

As shown in Fig. 4, the lead lining I5 forming the tank I4 is spacedapart from the casing IU in such a manner as to provide a duct 25 whichextends about all sides of the lead lining I5 inside the casing I0. Theduct 25 communicates with the interior of the tank I3, into which may beplaced a suitable temperature controlling fluid mass which will flowthrough the chamber I3 and around the duct 25 surrounding the lead,`lining I5 -of the tank I4. By varying the teme,

perature of thefluid medium in the tank I3, the temperatures within thetank I4 may b e controlled within definite predetermined limits.

'Ihe uid is fed into the tank I3 through a suitable conduit 26 which isoperatively connected with the source of uid supply and the level ismaintained in the tank I3 by means of a waste or outlet pipe 21 whichacts as an overiiow pipe when the fluid reaches the top thereof. Ifwater is used as the fluid temperature control medium in the tank I3,and its temperature is not sufficiently low to control the temperatureof the tank I4, a cake of ice or other refrigerant may be placed thereinand so refrigerate the iiuid which runs through the tank and around `thechamber 25. During the operation of the apparatus, the tank I3 functionsalso as a rinse tank and the aluminum or aluminum alloys or otherarticles which have been treated in the treating tank may be rinsed inthe tank I3 after the coating has been formed thereon.

Electrical energy is supplied to the tank I3 by means of a transformer3D which is suitably mounted on the framework II, preferably at a pointadjacentthe bottom of the tank I3. The transformer operates onalternating electrical current and in a preferred-embodiment has a ratedcapacity of 200 amperes and 24 volts. The transformer is of aconventional design adapted for use either on 110 volts or 220 volts,and is provided with a series of taps through which the A suitablecontrol panel 3l is electrically connected with the transformer 3i) andactsto provide the connections through which the current flows totheelectrolytic bath. Electrical connections are provided between thevtransformer 3i! through the control panel 3| with bus bars 32 whichextend crosswise of the tank M. The ar' ticlers to be treated, such forexample as aluminum or aluminum alloy pistons 33, are provided with asuitable clamp 34 which is electrically connected with and engages oneof the bus bars .32.110 suspend the article being treated into the.electrolyte in the tankV I4. In this manner the articles to be treated'are connected through the electrolytic bath in a parallel series sothat the flow of electricity is through the bus bars 32, the clamps 34,the article 33, through the electrolytic bath to the article 33, itsclamp 34, and the bus bar 32. This method of connection is contrarytovconventional methods ln that the tank or bath remains neutral,whereas in prior known processes the, bath or tank forms the otherconnection with the electrical current and only the article beingtreated forms the anode thereof. In the present apparatus, with the useof alternating current, the articles 33 alternately form the anode andthe cathode of the current flow and on the 11i) volt, 60 cycle, linethis alternation occurs sixty times each second, but will vary; however,with variations in the cycles of the a1ter,

nating current.

A valve 35 is provided in the casing I El and communicates with the duct25 to permit withdrawal of fluid temperature control mediums therefrom.

rThe foregoing described apparatuswhile being particularly adapted tothe process of the present invention, is also adaptable (with slightchanges, particularly as to its electrical circuit) for -use inconventional processes known to the art, and as such, by reason of thetemperature control provided in the electrolytic bath and in theportability of the unit, produces results which are desirable and whichare superior to the apparatus previously known to the art, although ltis my belief that using such apparatus with conventional processes willnot produce the satisfactory results produced by the practice of myimproved process therein.

'I'he improved process of the present invention proceeds in part upon mydiscovery that a superior type of coating may be formed either onaluminum or aluminum alloys or articles formed therefrom, and can beprovided by using the aluminum or aluminum alloy or article formedtherefrom as both the anode and the cathode of the electrical circuitflowing through the electrolytic bath. The change from anode to cath-,

ode is preferably a successively occurring change which will be broughtabout in relatively rapidly alternating sequence. The process alsoproceeds upon my discovery that such successively and rapidlyalternating electrical changes in the character of the aluminum oraluminum alloy during the coating operation can best be effected byutilizing various voltages of 60 cycle alternatihg current, beginning atapproximately 12.5

volts and increasing'the voltage of the electrical flow dring thecoating operation to 24 volts at a time when the temperature of the bathis deflnitely controlled between limits of approximately 69 to '74 F.

In carrying. out the present process, I provide an electrolytic bath inthe tank by using commercial grades of sulphuric acid in concentrationsof 13% by weight in aqueous solution. At least y two pieces of thealuminum or aluminum alloy or article formed therefrom are electricallyconnected, one with each side of the line supplying the electric current.to the electrolytic bath and from the anode and cathode respectively ofthe cell resulting from the immersion of the articles in theelectrolyte.

The operation is started at a voltage of the lower order, preferablyfrom 9 to 121/2 volts, which is increased as the operationprcceeds, tothe higher range `of voltages, preferably from 20 to 24 volts. Thetemperature of the bath is /preferably maintained from approximately 69F. to approximately 73 F. by the use of a suitable circulatingtemperature control medium surrounding the tank in which the operationis being carried on. The treatment and processing is complete Withintime intervals of about 20 to 40 minutes, the

operation being carried on'approximately threequarters of the time atthe higher range of voltages, i. e. voltages in excess of 12.5 volts.

In'coatings which are formed on the aluminum or aluminum alloy, or thearticles formed therefrom, I have found that there is relatively noprecipitation of the metal into the electrolytic bath such as occurs informing coatings by means of conventional processes utilizing directcurrent electricity. This feature of the present process permits the useof the electrolytic bath for a very much longer period of time, for oncethe acid concentration ofthe bath has-been normalized it appears tocontinue at the normalized acidconcentration during the entire operationfor a considerable period of time. ,Y Utilizing direct current, themetal decomposes yand reacts with the bath to form sulphates.

When the bathl contains 20 grams per litre, the bath must be completelychanged because 4of the resultant decrease in electrical conductivity ofthe bath. The present process is characterized by the fact that littleor none of the metal-is decomposed by the electrolyte and put into thebath in the form of va, sulphate. This is due to the use of therelatively dilute bath which is possible by the use of the alternatingcurrent electrical energy in the' process. This particular feature isone of theoutstax'iding defects of the direct current method now beingused, as this sulphate is insoluble in the bath and readily affects theconductivity of the bath, and when it reaches the concentration of 20grams per litre of electrolyte, the bath must be changed. V 'I'heefliciency of such processes of theart is progressively lessened by therapidly diminishing conductivity value in this bath up to the ultimatepoint of 20 grams per litre.

The coatings formed by the process of the present invention as abovedescribed, will be found.

to have a substantially increased uniformity of hardness throughout thesuccessive layers, which,

when investigated by means of photomicrographs, will be shown to possessa very ne grained crystalline structure and very small and uniformlydistributed pores extending therethrough. -In an average'4 case, thevariation between the softest layer and the hardest layer is intheneighborhood of approximately 20%, Whereas in a characteristic coatingformed by the processes of the prior art, utilizing direct currentelectrical energy and baths of the specied concentrations of acid, thereis a variation of from 140 microcharacter to approximately 5,000microcharacter between the hardest layer and the softest layer formed.

Also, there is a considerably thicker coating formed by means of theprocess herein disclosed than that of processes utilizing a ilow ofdirect current electrical energy. In a typical example, the coatingformed by the process herein described is approximately twice as thickas the coating formed by the conventional direct current processestreated for like periods of time. I believe this is due to the fact thatthe metal is not decomposed and changed to the sulphate as is the casein all prior operations.

A particular field in which the present invention has great utility isin the iield of formed protective and oil bearing coatings onaluminum oraluminum alloy pistons for use in internal com-- bustion engines. Inthis field the coatings of the present invention are -superiortocoatings formed by prior methods in that the coating is very much moreclosely knit, being formed of very much sions, other circumstances beingequal, are con-v versely proportional to the diameters of the capillarytubes so that, considering the pores of the coating as capillary tubeshaving an ainity for the oil or other uid medium, it will be seen -thatthe smaller size of the pores and the greater uniformity of placement ofthe pores in the coating resulting from the present invention, willaccount for the greatly increased capillary values of such coating.

Excellent oxide coatings are produced under the following conditions onpistons produced from a low-aluminum high-silicon alloy which containsapproximately 12% of silicon. An equal number of pistons are placed inthe electrolyte and act"4 as the electrodes. The electrolyte is composedof approximately 13% by weight of commercial sulphuric acid in aqueoussolution. A l2 volt alternating current` is used, one side of the linebeing connected to each electrode.and the .12..volt alternating currentis impressed'bn the "cell for a period ofV 3 minutes. The voltage isthen increased to 24 volts and the treatment is conto note that similarcoatings formeduby conyentional methods utilizing direct current and astronger electrolyte, where the article is treated for the same lengthof time, produces coatings having an average thickness of .000327 inchas against the average thickness of the coatings produced by the use ofthe alternating` current of .000746 inch; land the abrasimeter valueswill be less than 30. y

In treating the aluminum o r aluminum alloy articles by the4 presentprocess, I nd that it is possible to dispense with -the preliminarycleaning and Washing processes which are common to the prior art,itbeing sufficient with the improved process of the present invention tomerely dip thealuminum or 'aluminum alloy article in a hot water bath.This is possible, in my opinion,

conventional processes.

because of the use of the alternating current in the present processwhich, in conjunction with the electrolyte, reacts in such a way as toin eiiect clean the articles more completely and thoroughly than itwould be possible to do by previously suggested methods. This, Ibelieve., is due to some extent at least to the action oi thealternating current in expelling the adhered surface oxygen.

A characteristic of the coating produced is that it is very smooth onthe exterior surface so as to make possible the use of the articlewithout using a buiiing machine for the purpose of removing thechalk-like soft coating which is formed by the treatment of aluminum orits alloys by The soft outer coating formed by conventional processes isa coating which consists, to a large extent, of the material which isprecipitated into the bath as the sulphate which has heretofore beendescribed. The coating formed by the present process has no such softouter coating and tests to date indicate that there is not more thanapproximately 20% variation in the microcharacter val ues from theoutside skin of the coating to its base where it forms on the aluminumor the aluminum alloy.

Due to the action of the alternatingcurrent, less metal is taken awayfrom the material undergoing treatment and thus the coating is oigreater strength than coatings produced by prior methods wherein more ofthe metal is taken away during the processing operation.

While particular uses have been mentioned in the foregoing discussion ofthe apparatus and process of the present invention, it is to beunderstood that both the apparatus and the process are adapted to otheruses than those herein particularly mentioned and'may be usedsuccessfully to form protective coatings on articles which may bejformedof some material other than aluminum or aluminum alloy but which may becoated with aluminum or aluminum alloy by such processes as calorizing,metal spraying, etc.

I claim:

1 A process of producing a protective coating having a thickness lnexcess of approximately .0004 inch and a hardness in excess ofapproximately'30 gramsabraslmeter value, on an alux'ninum-'siliconalloywhich contains approximately 12% of lsilicon having substantiallyincreased uniformity of hardness throughout the successive layers, whichcomprises immersing the farticles'formed of vsaid alloy in anelectrolytic bath 'containing 13% by weight of sulphuric 'acid inaqueous solution, electrically connecting the immersed articles inparallel with 'a source of alternating current electrical energy and.utilizing the immersed articles as both the anode and cathode of thecircuit, and subjecting said articles to a flow of alternating currentelec trical energy in successive stages beginning at approximately 9 to12.5 volts intensity for a period of approximately 3 minutes durationand increasing through successive stages to approxi-r mately 24 voltsfor a period of approximately7 20 minutes duration at the conclusion ofthe process.

2. A process of producing a protective coatingv on an aluminum-siliconalloy containing approximately 12% silicon having substantiallyincreased uniformity of hardness throughout the successive layers, whichcomprises immersing the aluminum alloy in an electrolytic bathcontaining 13% by weight of sulphuric acid in aqueom solution,electricallyconnecting the immersed alloy in parallel with a source ofalternating current electrical energy and utilizing the immersed alloyas both the anode and cathode of the circuit through the bath, andsubjecting the said alloy to a flow of alternating current electricalenergyin successive stages beginning at approximately9 lto 12.5 'voltsfor a period of approximately 3 minutes and thereafter increasing toapproximately 20 to 24 volts for a period of approximately 20 minuteswhile agitating the bath to maintain a substantially constant circu-llation thereof and maintaining a substantially constant temperature ofthe bath between approximately 70 and 74 Fahrenheit.

. 3. A process for forming a nished and smooth unbuffed protectivecoating having a thickness in excess of .0004 inch and a, hardness inlexcess of approximately 30 grams abrasimeter value on analuminum-silicon alloy containing approximately 12% silicon havingsubstantially 1ncreased uniformity of hardness throughout the successivelayers, which comprises immersing said alloy in an-oxidizing bathcontaining 13% by weight of sulphuric acid in an aqueous solution andpassing an electrical current therethrough for a period of approximately3 minutes duration at a potential of approximately 9 to 12.5 volts andthereafter for a period of approximately 20 minutes duration at apotential of approximately 24 volts, utilizing the said alloy in thebath as both the anode and cathode of the circuit through the bath,while agitating the bath and maintaining therein a substantiallyconstant temperature of between approximately 70 a'ndf'l" Fahrenheit.

' FRED A. WALES.

